Bimetallic nanoparticle catalysts attract significant interest because of the unique electronic and structural characteristics of catalytically active mixed metal phases, which can confer synergistic enhancements over pure metals in the turnover frequency and selectivity of reactions as diverse as CO oxidation, alkane dehydrogenation, and NOx reduction. These enhancements in the rate and selectivity are often accompanied by further benefits for catalysis. The addition of a second metal may assist in the reduction of the first, improve the thermal stability of metals prone to cluster agglomeration, or preclude deactivation by sulfur or other poisons. Such effects may be brought forth by electronic modifications of the first metal with
the second, which lead to partial charges that can alter adsorbate binding characteristics, or through coupled but distinct geometric effects in which small ensembles of the first metal are isolated and stabilized by the diluting metal. Rigorous studies that attempt to clearly distinguish these effects or provide a holistic mechanistic interpretation of the reactivity of bimetallic clusters require nanoparticles that are uniformly distributed in size and composition.
Strategies to prepare such well-defined alloys, however, often face synthetic challenges that preclude the achievement of these stringent requirements for cluster uniformity or may achieve this uniformity only at the expense of general applicability to clusters of diverse elemental composition.
Bimetallic clusters are most commonly prepared through the sequential adsorption and precipitation or co-impregnation of metal salts onto mesoporous scaffolds. Such techniques, however, suffer from an inability to carefully control the placement of the metals onto the support, and thus lead to bimodal mixtures of monometallic and bimetallic species. Controlled assembly techniques resolve these shortcomings through sequential grafting of organometallic compounds, first onto an oxide support and then onto the covalently anchored metal itself, where the latter step enforces strong metal-metal interactions that promote alloying. These techniques are limited to metals and metal complexes that selectively interact with each other instead of the support, and often form monometallic clusters of the second deposited metal. Galvanic displacement and electroless deposition methods, by contrast, allow the selective placement of a secondary metal onto pre-formed monometallic clusters through redox chemical reactions. These techniques typically result in bimetallic clusters uniformly distributed in composition, though their dispersion is ultimately limited to that of the monometallic seeding metal; the elements available for deposition onto these seeds are also restricted to metals with precursors stable against homogeneous nucleation of their monometallic clusters. Colloidal synthesis techniques, which typically proceed via the reduction of metal cation precursors in the presence of polymers that prevent agglomeration of the suspended nanoparticles, can produce bimetallic clusters that are uniformly distributed in composition and highly dispersed in size. The removal of the attached polymers, however, often requires treatment at elevated temperatures (>573 K), which can lead to sintering processes that compromise the intended size and compositional uniformity of the bimetallic clusters.
Alloy nanoparticles can alternatively be prepared within the voids of zeolite materials. Confinement within such voids leads to several and additional and distinct advantages for catalysis, including the protection of active metal surfaces from large poison species, the stabilization of specific transition states, and the reactant size selection properties that have made zeolites such ubiquitously useful catalysts. Metal encapsulation within zeolitic voids is achieved through the ion exchange of cationic metal precursors onto negatively charged sites in zeolite frameworks. Reductive treatment of these exchanged zeolites forms monometallic clusters dispersed throughout the zeolitic voids, after which the exchange and reduction of a second metal forms encapsulated bimetallic clusters. Such techniques have been successfully implemented to prepare encapsulated alloy clusters stable against sintering at temperatures in excess of 573 K, although the successive ion exchange process does not guarantee uniform compositions and is limited to zeolites with pore apertures wide enough for solvated metal cations to enter the framework. The apertures within small-pore and medium-pore zeolites preclude post-synthetic encapsulation protocols via ion-exchange from aqueous media, which require the migration of solvated metal-oxo oligomers that cannot diffuse through the small apertures in such zeolites.
According to the present disclosure, bimetallic clusters narrowly distributed in size and composition have now been encapsulated within the voids of zeolites that preclude post-synthetic encapsulation protocols via a ligand-assisted hydrothermal synthesis technique.